1. Field of the Invention
This invention relates to a device for producing quantum effects, namely a fiber that is capable of carrying energy with an exterior surface populated by quantum dot structures that are controlled by changes in the energy carried by the fiber. The invention has particular, but not exclusive, application in materials science as a programmable dopant that can be placed inside bulk materials and controlled by external signals.
2. Description of the Related Art
The fabrication of very small structures to exploit the quantum mechanical behavior of charge carriers (e.g., electrons or electron “holes”) is well established. Quantum confinement of a carrier can be accomplished by a structure whose linear dimension is less than the quantum mechanical wavelength of the carrier. Confinement in a single dimension produces a “quantum well,” and confinement in two dimensions produces a “quantum wire.”
A quantum dot (QD) is a structure capable of confining carriers in all three dimensions. Quantum dots can be formed as particles, with a dimension in all three directions of less than the de Broglie wavelength of a charge carrier. Such particles may be composed of semiconductor materials (including Si, GaAs, InGaAs, InAlAs, InAs, and other materials), or of metals, and may or may not possess an insulative coating. Such particles are referred to in this document as “quantum dot particles.” A quantum dot can also be formed inside a semiconductor substrate, through electrostatic confinement of the charge carriers. This is accomplished through the use of microelectronic devices of various design, e.g., a nearly enclosed gate electrode formed on top of a quantum well, similar to a P-N-P junction. Here, the term “micro” means “very small” and usually expresses a dimension less than the order of microns, i.e., thousandths of a millimeter. The term “quantum dot device” refers to any apparatus capable of generating a quantum dot in this manner. The generic term “quantum dot,” abbreviated QD in certain drawings in this application, refers to any quantum dot particle or quantum dot device.
The electrical, optical, thermal, magnetic, mechanical, and chemical properties of a material depend on the structure and excitation level of the electron clouds surrounding its atoms and molecules. Doping is the process of embedding precise quantities of carefully selected impurities in a material in order to alter the electronic structure of the surrounding atoms, for example, by donating or borrowing electrons from them, and therefore altering the electrical, optical, thermal, magnetic, mechanical, or chemical properties of the material. Doping levels as low as one dopant atom per million atoms of substrate can produce measurable changes in the expected behavior of the pure material, for example, by altering the band gap of a semiconductor.
Quantum dots can have a greatly modified electronic structure from the corresponding bulk material, and therefore different properties. Quantum dots can also serve as dopants inside other materials. Because of their unique properties, quantum dots are used in a variety of electronic, optical, and electro-optical devices.
Kastner, “Artificial Atoms,” Physics Today (January 1993) points out that the quantum dot can be thought of as an “artificial atom,” since the carriers confined in it behave similarly in many ways to electrons confined by an atomic nucleus. The term “artificial atom” is now in common use, and is often used interchangeably with “quantum dot.”However, for the purposes of this document, “artificial atom” refers specifically to the pattern of confined carriers (e.g., an electron gas), and not to the particle or device in which the carriers are confined.
The term “programmable dopant fiber” refers to a wire or fiber with quantum dots attached to, embedded in, or formed upon its outer surface. This should not be confused with a quantum wire, which is a structure for carrier confinement in two dimensions only.
Quantum dots are currently used as near-monochromatic fluorescent light sources, laser light sources, light detectors (including infra-red detectors), and highly miniaturized transistors, including single-electron transistors. They can also serve as a useful laboratory for exploring the quantum mechanical behavior of confined carriers. Many researchers are exploring the use of quantum dots in artificial materials, and as programmable dopants to affect the optical and electrical properties of semiconductor materials.
Kastner describes the future potential for “artificial molecules” and “artificial solids” composed of quantum dot particles. Specifics on the design and functioning of these molecules and solids are not provided. Leatherdale et. al., Photoconductivity in CdSe Quantum Dot Solids,” Physics Review B (15 Jul. 2000) describe, in detail, the fabrication of “two- and three-dimensional . . . artificial solids with potentially tunable optical and electrical properties.” These solids are composed of colloidal semiconductor nanocrystals deposited on a semiconductor substrate. The result is an ordered, glassy film composed of quantum dot particles, which can be optically stimulated by external light sources, or electrically stimulated by attached electrodes, to alter its optical and electrical properties. However, these films are extremely fragile, and are “three dimensional” only in the sense that they have been made up to several microns thick. In addition, the only parameter that can be adjusted electrically is the average number of electrons in the quantum dots. Slight variations in the size and composition of the quantum dot particles mean that the number of electrons will vary slightly between dots. However, on average the quantum dot particles will all behave similarly.
The embedding of metal and semiconductor nanoparticles inside bulk materials (e.g., lead particles in leaded crystal) is also well established. These nanoparticles are quantum dots with characteristics determined by their size and composition. They also serve as dopants for the material in which they are embedded to alter selected optical or electrical properties. However, there is no means or pathway by which these quantum dot particles can be stimulated electrically. Thus, the doping characteristics of the quantum dots are fixed at the time of manufacture and cannot be adjusted thereafter.
In general, the prior art almost completely overlooks the broader materials-science implications of quantum dots. The ability to place programmable dopants in a variety of materials implies a useful control over the bulk properties of these materials. This control could take place not only at the time of fabrication of the material, but also in real time, i.e., at the time of use, in response to changing needs and conditions. However, there is virtually no prior art discussing the use, placement, or control of programmable quantum dots in the interior of bulk materials. Similarly, there is no prior art discussing the placement of quantum dots on the surface of an electrically or optically conductive fiber.
U.S. Pat. No. 5,881,200 to Burt (1999) discloses an optical fiber (1) containing a central opening (2) filled with a colloidal solution (3) of quantum dots (4) in a support medium. (See prior art FIGS. 1 and 2.) The purpose of the quantum dots is to produce light when optically stimulated, for example, to produce optical amplification or laser radiation. The quantum dots take the place of erbium atoms, which can produce optical amplifiers when used as dopants in an optical fiber. This fiber could be embedded inside bulk materials, but could not alter the properties of such materials since the quantum-dot dopants are enclosed inside the fiber. In addition, no means is described for exciting the quantum dots electrically. Thus the characteristics of the quantum dots are not programmable, except in the sense that their size and composition can be selected at the time of manufacture.
U.S. Pat. No. 5,889,288 to Futatsugi (1999) discloses a semiconductor quantum dot device that uses electrostatic repulsion to confine electrons. This device, as shown in prior art FIGS. 3a and 3b consists of electrodes (16a, 16b, and 17) controlled by a field effect transistor, both formed on the surface of a quantum well on a semi-insulating substrate (11). This arrangement permits the exact number of electrons trapped in the quantum dot (QD) to be controlled, simply by varying the voltage on the gate electrode (G). This is useful, in that it allows the “artificial atom” contained in the quantum dot to take on characteristics similar to any natural atom on the periodic table, and also to transuranic and asymmetric atoms which cannot easily be created by other means. Unfortunately, the two-dimensional nature of the electrodes means that the artificial atom can exist only at or near the surface of the wafer, and cannot serve as a dopant to affect the wafer's interior properties.
Turton, “The Quanturn Dot,” Oxford University Press (1995) describes the possibility of placing quantum dot devices in two-dimensional arrays on a semiconductor microchip, explicitly as a method for producing new materials through programmable doping of the substrate. This practice has since become routine, although the spacing of the quantum dot devices is typically large enough that the artificial atoms formed on the chip do not interact significantly, nor produce macroscopically significant doping. Such a chip also suffers from the limitation cited in the previous paragraph: its two-dimensional structure prevents its being used as a dopant except near the surface of a material or material layer.
Goldhaber-Gordon et at., “Overview of Nanoelectronic Devices,” Proceedings of the IEEE, v. 85, n.4 (April 1997) describe what may be the smallest possible single-electron transistor. This consists of a “wire” made of conductive C6 (benzene) molecules, with a “resonant tunneling device” or RTD inline that consists of a benzene molecule surrounded by CH2 molecules that serve as insulators. The device is described (incorrectly, we believe) as a quantum well rather than a quantum dot, and is intended as a switching device (transistor) rather than a confinement mechanism for charge carriers. However, in principle the device should be capable of containing a small number of excess electrons and thus form a primitive sort of artificial atom. Thus, the authors remark on page 532 that the device may be “much more like a quantum dot than a solid state RTD.” The materials science implications of this are not discussed.
McCarthy, “Once Upon a Matter Crushed,” Science and Fiction Age (July 1999), in a science fiction story, includes a fanciful description of “wellstone,” a form of “programmable matter” made from “a diffuse lattice of crystalline silicon, superfine threads much finer than a human hair,” which use “a careful balancing of electrical charges” to confine electrons in free space, adjacent to the threads. This is probably physically impossible, as it would appear to violate Coulomb's Law, although we do not wish to be bound by this. Similar text by the same author appears in McCarthy, The Collapsium,” Del Rey Books (August 2000) and McCarthy, “Programmable Matter,” Nature (5 Oct. 2000). Detailed information about the composition, construction, or functioning of these devices is not given.